1. Field of the Invention
The present invention relates to a liquid crystal display element to be used as a display device for, e.g., television sets, personal computers, word processors, and OA (Office Automation) apparatuses. In particular, the present invention relates to a liquid crystal display element which has a reflection type display mode.
2. Description of the Related Art
As disclosed, for example, in Japanese Laid-Open Publication No. 9-311351, a conventional liquid crystal display element has a pair of insulative substrates usually made of glass. Active elements such as TFTs (thin film transistors) are placed on one of the substrates as switching elements for controlling the electro-optical characteristics of the liquid crystal. Gate signal lines receiving a driving signal for driving the switching elements and source signal lines receiving a display signal are disposed in such a manner that they intersect each other.
Furthermore, on the substrate having TFTs (hereinafter, referred to as an xe2x80x9cactive matrix substratexe2x80x9d), an inter-layer insulation film is formed over the TFTs and both signal lines, and pixel electrodes are further placed on the insulation film so that the pixel electrodes overlap with the TFTs and the signal lines. By constructing such a structure, light entering any region in which a signal voltage is not applied to the liquid crystal is blocked by both signal lines. Therefore, it becomes unnecessary to provide a black matrix (hereinafter, referred to as a xe2x80x9cBMxe2x80x9d) which has been conventionally provided in a display region of the other substrate (hereinafter, referred to as a xe2x80x9cCF substratexe2x80x9d) on which a color filter is provided. Moreover, it is known that, in this structure, the entire portion excluding the gate signal lines, source signal lines, and TFTs can be utilized as display pixels, thereby improving the opening ratio of the liquid crystal display element.
Japanese Laid-Open Publication No. 10-62768 discloses a structure, as an implementation of the liquid crystal display element disclosed in the above mentioned Japanese Laid-Open publication No. 9-311351, in which a light-blocking member is provided between gate signal lines and source signal lines on an active matrix substrate so as to block light without providing a BM in a frame region on the outer periphery of a display region. In this structure, an increase in production steps resulting from the light-blocking member can be avoided by forming the portion of the light-blocking member between the source signal lines from the same material that is used for the gate signal lines, and the portion of the light-blocking member between the gate signal lines from the same material that is used for the source signal lines. Furthermore, color layers of a color filter on a CF substrate are formed so as to extend to the frame region, thereby concealing the wiring pattern and the like on the active matrix substrate.
In accordance with the above-described conventional technique, it is possible to produce a CF substrate from only three layers of R (red color), G (green color), and B (blue color), instead of the conventionally-required four layers, namely R, G, B and a BM (black matrix) thereby enabling a significant reduction in the production cost of the CF substrate.
The active matrix substrate and the CF substrate are usually attached together with a sealing material. When attaching, spacers are placed within the sealing material and on either one of the active matrix substrate and the CF substrate in order to provide a predetermined gap between the active matrix substrate and the CF substrate. In the case of a TN-mode liquid crystal display element, the gap between the two substrates is usually about 4 xcexcm to about 6 xcexcm, with a variation of about xc2x110%. Then a liquid crystal is injected by vacuum injection through an injection hole provided in a portion of the sealing material. By closing the injection hole with an UV setting resin, the liquid crystal display element is accomplished.
The above-described conventional technique, which is directed toward transmission type liquid crystal display elements having pixel electrodes formed of a transparent conductive material (i.e., a conductive material having a relatively high transmittance), provides a method for producing a CF substrate having only three layers of R, G, and B, while a BM is provided on the active matrix substrate. As a result, the production cost of the liquid crystal display element is significantly reduced.
However, in the case of a reflection type liquid crystal display element, pixel electrodes made of a reflective conductive material (i.e., a conductive material having a relatively high reflectance) are formed on the active matrix substrate, and images are displayed by controlling the reflection of light entering the surface of the liquid crystal display element. For this reason, it is necessary to suppress the reflection of light in the region which is irrelevant to the display function. Conventionally, suppression of such reflection of light has been accomplished by providing a BM which is composed of a light absorption film or a low-reflectance film on the CF substrate. Thus, in order to construct a CF substrate which does not include a BM but only includes three layers of R, G, and B, or complementary colors of C (cyan), M (magenta), and Y (yellow), it is essential to consider how to suppress the reflection of light in the regions irrelevant to the display function.
In the display region, gate signal lines receiving a driving signal for driving TFTs and source signal lines receiving a display signal are inevitably noticeable between adjacent pixel electrodes. Thus, the first problem to be solved is how to suppress light reflection on these signal lines.
The second problem to be solved is how to suppress light reflection on each signal line in the frame region on the outer periphery of the display region.
The third problem to be solved is how to protect the TFT elements from external light. If light energy enters a channel layer of a TFT element, a leakage current (photo-leakage current) is generated in an off-state of the TFT. This prevents a sufficient voltage from being applied to liquid crystal layer, and prevents the TFT element from displaying images properly. However, a reflection type liquid crystal display element is expected to operate under a maximum illuminance of 100,000 lx in the direct sunlight. In other words, a reflection type liquid crystal element may be subjected to light having a maximum of one hundred fold energy as compared to the 10,000 lx which a conventional transmission type liquid crystal display element may be subjected to, or the 1,000 lx (light energy commonly observed in an office during the daytime) which most-widely-used notebook type PCs may be subjected to. Thus, the TFT elements must be protected from light by a material whose ability to block light is equal to or greater than that of a conventional BM.
According to one aspect of the invention, a liquid crystal display element includes an active matrix substrate; a color filter substrate; a sealing material for attaching the active matrix substrate and the color filter substrate with a predetermined gap maintained therebetween; and liquid crystal injected in the gap between the active matrix substrate and the color filter substrate. The active matrix substrate includes a pixel electrode comprising a reflective conductive material; a switching element connected to the pixel electrode; a gate signal line receiving a driving signal for driving the switching element; and a source signal line receiving a display signal. A plurality of said pixel electrodes and a plurality of said switching elements are arranged in a matrix, and a plurality of said gate signal lines and a plurality of said source signal lines are arranged so as to intersect each other. The color filter substrate includes a color filter having a plurality of color layers corresponding to a plurality of colors; a display region; and a frame region positioned on the outer periphery of the display region, the plurality of color layers being formed in the display region and in the frame region.
In one embodiment of the invention, said pixel electrode is formed so as to cover said switching element.
In another embodiment of the invention, the plurality of color layers include a red color layer, a green color layer, and a blue color layer; and at least one pair of color layers selected from the red color layer and the green color layer; the red color layer and the blue color layer; and the green color layer and the blue color layer is deposited at a position corresponding to the switching element.
In still another embodiment of the invention, at least one of said gate signal line and said source signal line comprises a transparent conductive film.
In still another embodiment of the invention, one or more further layer is deposited on at least one of said gate signal line and said source signal line including the transparent conductive film. The one or more further layer comprises at least one film selected from a light-transmissive oxide film and a light-transmissive nitride film.
In still another embodiment of the invention, adjacent color layers among the plurality of color layers are deposited at a position corresponding to at least one of said gate signal line and said source signal line.
In still another embodiment of the invention, the plurality of color layers formed in the frame region are arranged so as not to overlap the sealing material.
In still another embodiment of the invention, the plurality of color layers formed in the frame region are arranged so as to overlap the sealing material, the sealing material having a thickness of equal to or greater than about 5 xcexcm.
In still another embodiment of the invention, the plurality of color layers formed in the frame region are arranged so as to overlap the sealing material, an overlapping width between the plurality of color layers and the sealing material accounting for less than about 50% of a width of the sealing material.
In still another embodiment of the invention, the sealing material comprises a thermosetting resin and includes a first portion in which the sealing material does not overlap with the plurality of color layers formed in the frame region, the first portion having a thickness of equal to or greater than about 5 xcexcm.
In still another embodiment of the invention, the plurality of color layers formed in the frame region and the plurality of color layers formed in the display region are constructed in the same sequence of colors and in the same pitch.
In still another embodiment of the invention, said pixel electrode further comprises a transparent conductive material.
In still another embodiment of the invention, the active matrix substrate further includes a light-blocking member.
In still another embodiment of the invention, the light-blocking member is provided in a region having an area wider than a region where the plurality of color layers formed in the frame region are provided.
According to the present invention, in a liquid crystal display element having a reflection type display mode, a CF substrate can be formed only with three layers of R, G, and B or complementary colors of C, M, and Y, while omitting a BM (which would be conventionally provided for suppressing light reflection in regions irrelevant to the display function). Furthermore, according to the present invention, it is possible to achieve suppression of the reflection of light in the regions irrelevant to the display function, (as would conventionally have been attained by a BM), as well as protecting switching elements from external light.
With respect to the aforementioned first problem to be solved, as disclosed in e.g., Japanese Laid-Open Publication No. 9-292698, a light-transmissive nitride film (e.g., TaN) may be provided on a metal wiring material (e.g., Ta) forming the gate signal lines and the source signal lines, or a transmissive oxide film (e.g., CrO) may be provided on a metal wiring material (e.g., Cr), so as to reduce the light reflectance on each signal line. Alternatively, the gate signal lines and the source signal lines can be formed of a transparent conductive film (e.g., ITO (Indium Tin Oxide)), thereby reducing the reflectance of the lines themselves. In such a structure, the gate signal lines and the source signal lines positioned between adjacent pixel electrodes can be prevented from being noticeable.
Furthermore, if necessary, the color layers of the CF substrate can be positioned in such a manner that adjacent color layers overlap with each other on each signal line. As a result, light which is incident on each signal line is reduced so that the reflection of that light can in turn be lowered.
The above-described measure may not necessarily be required for reflection type liquid crystal display element for the following reasons: A reflection type liquid crystal display element commonly employs a display mode in which a black image is displayed with the application of an electric field. Accordingly, in a reflection type liquid crystal display element, a voltage which is equal to or greater than about xe2x88x9210 V is always applied to the gate signal lines in a non-writing status, and a voltage of about +6 V is always applied to the gate signal lines in a writing status. As a result, the portions above the gate signal lines are substantially displaying a black image, and thus, the above-described measure may not necessarily be required.
Next, with respect to the above-described second problem to be solved, the light reflection in the frame region is reduced by forming color layers in the frame region of the CF substrate. Such color layers in the frame region are patterned using the same material used for the color layers of the color filter provided in the display region, simultaneously with that color layer. Accordingly, no additional production step is required. In this case, it is preferable to omit a light-blocking member as disclosed in Japanese Laid-Open Publication No. 10-62768 because such a light-blocking member, provided on the active matrix substrate in order to block light entering a region between the gate signal lines and the source signal lines, would result in an undesirable increase in the reflectance.
Furthermore, as described above, by providing an oxide film or a nitride film on the metal wiring material, or by reducing the reflectance on the signal lines with the use of a transparent conductive film, the light reflection can be further decreased.
Lastly, the above-described third problem can be solved by providing pixel electrodes in such a manner as to cover the switching elements (TFTs). Since Al or Ag, both of which have a high reflectance, is used for pixel electrodes in a reflection type liquid crystal display element, the light irradiating the TFT can be reduced to a level on the order of several %, thereby enabling protection of the TFTs from the external light.
Moreover, since a channel layer of a TFT is sensitive to light having a short wavelength, it becomes possible to suppress the photo-leakage current by providing at least G or R on the TFT (in the case where the CF substrate is formed of color layers of R, G, and B).
However, the aforementioned technique of addressing the second problem, which involves forming a color layer which is the same as that of color filters in the frame region may result in the following problems:
Generally, in a liquid crystal display element, a thermosetting resin or a UV setting resin is employed as a sealing material. As a method for coating such a resin on a sealing structure, a screen printing method, a relief printing method, a dispenser coating method or the like can be used. In any one of these methods, the viscosity of the resin material is adjusted so as to obtain an optimum level of viscosity for coating. Such viscosity adjustment is usually achieved by adding silicon oxide or alumina having a particle diameter of about 1 xcexcm to about 3 xcexcm, referred to as a filler. The resin is applied on either the active matrix substrate or the CF substrate by using the above-described coating method, with spacers (diameter: about 5 xcexcm) being employed for maintaining a predetermined gap (i.e., cell thickness) for the liquid crystal layer.
On the other substrate, usually, spacers having a diameter of about 5 xcexcm are spread in the amount of about 100 units/mm2 to maintain a uniform gap at the portion of the liquid crystal layer which corresponds to the display region. The pair of substrates are then aligned, with a sufficient load being applied to retain the predetermined gap (note that the load depends on the size of the substrate, the area and viscosity of the sealing resin, and the like). Then, the substrates are subjected to a heat treatment or UV ray application, depending on the curing conditions of the particular sealing material used.
Since the above-described filler and spacers are mixed as additives in the sealing material, if the load application is performed under inappropriate conditions (e.g., if the application time before reaching a predetermined load is inadequate (i.e., inadequate load application speed), or if the thickness of the sealing material is extremely compressed to about twice the filler diameter or less), a phenomenon may be observed in which a low-viscosity resin component within the sealing material is separated from the additives and begins to flow.
Moreover, in the case of a thermosetting resin, which sets in response to the application of heat, the resin first reaches its softening point before the setting reaction starts, so that the viscosity of the resin material may drastically decrease at the softening point. Thus, a thermosetting resin is even more susceptible to the separation phenomenon of the above-described resin component and the additives. The separation phenomenon is not in itself a direct cause of a defective liquid crystal display element, but, when the separated resin component flows out to the display region, it may occasionally result in a defect.
In the present invention, by constructing the frame region and the sealing material so that they overlap with each other, the thickness of the sealing material can be reduced as compared to that of the conventional liquid crystal display element in which a BM is provided in the frame region of the CF substrate. For example, a conventional liquid crystal display element typically incorporates a BM which is formed of a metal material such as Cr to a thickness of about 0.3 xcexcm, whereas the color layer provided according to the present invention is about 1.5 xcexcm thick, resulting in a difference of 1.2 xcexcm in the sealing material. As will be appreciated, a thinner sealing material may more readily induce the above-described problem of extreme compression.
Moreover, a reflection type liquid crystal display element has an optical path length which is twice an optical length of a transmission type liquid crystal display element. Accordingly, the cell thickness is required to be reduced to xc2xd of that of a transmission type liquid crystal display element, which in turn reduces the thickness of the sealing material by about xc2xd. This induces the separation phenomenon of the resin component of the sealing material and the additives, as illustrated above.
Therefore, in order to prevent such a phenomenon, in which a low-viscosity resin component within the sealing material is separated from the additives and begins to flow at the portion where the color layer of the frame region and the sealing material overlap, the present invention prescribes a defined range of thicknesses of the sealing material and a defined range of overlap ratios of the color layer in the frame region with respect to the width of the sealing material.
Specifically, the color layer in the frame region and the sealing material are arranged so as not to overlap with each other at all. Alternatively, even if the color layer in the frame region and the sealing material made of thermosetting resin overlap with each other, it is ensured that the thickness of the sealing material in the overlapping region is equal to or greater than about 5 xcexcm. Alternatively, it is ensured that the overlapping width between the color layer in the frame region and the sealing material is lower than about 50% of the width of the sealing material, while ensuring that the thickness of the sealing material in the non-overlapping region is equal to or greater than about 5 xcexcm. As a result, the defects in the sealing material can be minimized.
The color layers in the above-described frame region are preferably formed so as to have the same sequence of colors and the same pitch as those of the color filter in the display region. As a result, since the color arrangement becomes uniform in the frame region, the appearance of the display region can be further improved.
A liquid crystal display element can also be structured in such a manner that the above-described pixel electrodes are formed of at least two or more materials, such as a transparent conductive material (conductive material with a relatively high transmittance) and a reflective conductive material (conductive materials with a relatively high reflectance), thereby providing a liquid crystal display element having two or more types of display modes, i.e., a transmission mode and a reflection mode.
When a liquid crystal display element has a transmission mode and a reflection mode, it is preferable in the transmission mode that a light-blocking member be provided on the active matrix substrate especially in the frame region, as disclosed to Japanese Laid-Open Publication No. 10-62769.
Furthermore, by providing the light-blocking member in a region having a wider area than that of the color layer in the frame region on the above-described CF substrate, it is possible to further block the light from a back light means which enters obliquely due to the thickness of the glass insulative substrate.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display element having a reflection type display mode, in which the CF substrate is formed only with three layers of R, G, and B or alternatively with three complementary layers of C, M, and Y, thereby making it possible to omit a BM as in the case of a transmission type liquid crystal display element, so as to realize a significant reduction in the production cost.